Recently, a hybrid vehicle and an electric vehicle are drawing public attention as an eco-friendly vehicle. The hybrid vehicle is a vehicle having a motor driven not only by a conventional engine but also by a DC power source via an inverter as a mechanical power source. That is, the hybrid vehicle is to obtain the mechanical power source by driving the engine and converting DC voltage from the DC power source into AC voltage by the inverter so as to rotate the motor by the converted AC voltage.
The electric vehicle is a vehicle having a DC power source, an inverter and a motor driven by the inverter as the mechanical power source.
As a drive motor of such a hybrid vehicle and an electric vehicle, a permanent magnet type motor provided with a permanent magnet in a rotor and rotated by a magnetic action of poles due to the permanent magnet and a rotating magnetic field generated in a stator, a field winding type motor provided with a field coil in a rotor and rotated by a magnetic action of a component in the vertical direction to a rotation shaft of a magnetic field generated in the rotor by a field current passing through the field coil and a rotating magnetic field generated in a stator, and the like are taken into consideration.
For example, Japanese Patent Laying-Open No. 2004-229461 discloses a charge control device including a power generator having a field winding and plural-phase power generating winding, a rectifying circuit connected to a voltage output terminal of the power generator, a field current control switching element for controlling an electric current passing through the field winding of the power generator, connection switch means for switching and connecting the output side of the rectifying circuit to one of a high pressure storage cell and a low pressure storage cell, and control means for controlling ON/OFF of the field current control switching element and controlling switch of the connection switch means so as to charge the high pressure storage cell and the low pressure storage cell by respective reference voltage.
Here, torque generated from a motor is generally determined by magnetic flux passing from a rotor to a stator and an armature current passing through a stator winding. However, in the permanent magnet type motor, the magnetic flux between the rotor and the stator is determined by a permanent magnet to be used and maintained constant irrespective of the revolution speed. Therefore, for example as disclosed in Japanese Patent Laying-Open No. 5-304752, an AC electric motor for an electric vehicle for variably controlling content of magnetic flux by generating the magnetic flux by both a permanent magnet and a field pole provided on the rotor side is proposed.
According to the AC electric motor for the electric vehicle described in Japanese Patent Laying-Open No. 5-304752, the AC electric motor is a synchronous electric motor having the field pole on the rotor side, the permanent magnet is buried and an exciting coil is provided in the field pole. Then, field magnetic flux is generated by magnetic flux due to the permanent magnet and magnetic flux due to the exciting coil excited as required.
Further, in the hybrid vehicle and the electric vehicle, there is also consideration that the DC voltage from the DC power source is boosted by a voltage boosting converter and the boosted DC voltage is supplied to the inverter for driving the motor.
For example, Japanese Patent Laying-Open No. 2005-143157 discloses a motor control device for a vehicle including a field winding motor provided in the vehicle for generating the mechanical power, a battery, a voltage boosting and step-down inverter provided between the battery and the field winding motor, and a controller for performing field control and voltage boosting and step-down control in accordance with an operation state of the vehicle.
According to the above, the voltage boosting and step-down inverter is formed by an inverter unit, a voltage boosting and step-down unit and a capacitor. The voltage boosting and step-down inverter supplies electric power from the battery to a field winding motor for starting up an engine (a start-up motor), and supplies the electric power from the voltage boosting and step-down unit to field windings of the start-up motor and a field winding motor for driving rear wheels (a rear wheel drive motor). In this case, the controller performs control of supplying the electric power to the field windings by turning a field control circuit ON/OFF.
However, according to the AC electric motor for the electric vehicle described in Japanese Patent Laying-Open No. 5-304752, there is a need for a power circuit for supplying an exciting current to the exciting coil. In Japanese Patent Laying-Open No. 5-304752, a rotation transformer and a rectifier are provided in the interior of the electric motor and the exciting current is supplied from the exterior via the rotation transformer. Therefore, a physical constitution of the entire electric motor is enlarged in the axial direction relative to a main body of the electric motor and size and weight are not easily reduced.
In the motor control device for the vehicle described in Japanese Patent Laying-Open No. 2005-143157, there is also a need for a field control circuit for controlling power supply to the field windings. Therefore, there are problems that a physical constitution of the device is enlarged and cost is increased. These are against a demand of reducing size in the physical constitution of the device currently being increased from a view of ensuring interior space of the vehicle.
Therefore, this invention was made in order to solve the above problems and an object thereof is to provide a drive device of an electric motor capable of achieving both size reduction and higher output of the electric motor.